Commercially available ion implantation systems employ an ion source that includes a source chamber spaced from an implantation chamber where one or more workpieces are treated by ions from the source. An exit opening in the source chamber allows ions to exit the source so they can be shaped, analyzed, and accelerated to form an ion beam. The ion beam is directed along an evacuated beam path to the ion implantation chamber where the ion beam strikes one or more workpieces, typically circular wafers that have been placed within the implantation chamber. The energy of the ion beam is sufficient to cause ions which strike the wafers to penetrate those wafers in the implantation chamber. In a typical application of such a system the wafers are silicon wafers and the ions are used to `dope` the wafers to create a semiconductor material. Selective implantation with the use of masks and passivation layers allows an integrated circuit to be fabricated with such a prior art implanter. The equipment for this implantation technique is large, complex, expensive, and limited in its ability to implant ions at low energies.
U.S. Pat. No. 4,764,394 to Conrad entitled "Method and Apparatus for Plasma Source Ion Implantation" discloses an ion implantation system for treating a target by means of ionic bombardment. Ion implantation into surfaces of a three dimensional target is achieved by forming an ionized plasma about the target within an enclosing chamber. Once the plasma is set up in a region surrounding the target, ions from the plasma are driven into the target object from all sides without need to scan the target trough an ion beam. This implantation is accomplished by applying repetitive high voltage negative pulses to the one or more target workpieces. These pulses cause the ions to be driven into exposed surfaces of the target. A technique discussed in the '394 patent for setting up the plasma is to introduce a neutral gas into the region of the target and then ionize the gas with ionizing radiation. The system disclosed in the '394 patent to Conrad sets up the plasma in a region surrounding the workpiece and then selectively pulses an electrode that supports the workpiece to attract the charged ions in the plasma to the workpiece.
Theoretical studies of the plasma implementation process by Lieberman indicate that when a sudden negative voltage is applied to a target workpiece, within nanoseconds electrons near the surface are driven away from a region surrounding the workpiece, leaving a uniform density ion matrix sheath. M. A. Lieberman, "Model of Plasma Immersion Ion Implantation," J. Applied Physics, 66 (1989) p. 2926. Subsequently, on the time scale of the inverse ion plasma frequency, ions within the sheath are accelerated into the one or more workpieces. This in turn, drives the sheath boundary further from the target workpiece, exposing more ions which are driven into the workpiece. On a larger time scale, a steady state ion space charge condition typically develops. Sheathes of about one centimeter thickness are developed within microseconds. See, e.g., M. M. Shamim et al, "Measurement of Electron Emission due to Energetic Ion Bombardment in Plasma Source Ion Implantation," J. Applied Physics, 70 (1991) p. 4756.
The electric field within the plasma is low and substantially all of the potential applied to the target to accelerate the ions exists across the sheath. Typically, secondary electrons are produced as ions from the plasma strike the target surface. These electrons are accelerated from the target through the voltage drop across the sheath and terminate at the walls of the enclosing chamber. For a general discussion of secondary emission coefficients, see, e.g., S. Qin et al, "The Response of a Microwave Multipolar Bucket Plasma to a High Voltage Pulse with Finite Rise Time," IEEE Trans. Plas., Sci, 20 (1992) p. 569.
Some ion implantation systems, such as disclosed in Conrad, maintain the implantation chamber at a ground potential while pulsing the target workpiece with a relatively negative potential. For a substantially flat target workpiece requiring treatment on one face, such as a semiconductor wafer or a flat panel display substrate, the negatively pulsed target ion implantation system may be unfavorable. In such ion implantation systems, the target workpiece typically has to be raised to a high negative potential. This makes handling of the workpiece more difficult and complicates monitoring the potential delivered to charge collectors at the target workpiece by such monitoring devices as Faraday cups or calorimeters, since these devices also experience the high negative potential.